Oral thrush, a mucosal biofilm infection, continues to afflict an unacceptably high percentage of immunocompromised individuals, particularly neonates and patients on immunosuppressive medications. Although C. albicans is the primary etiologic pathogen, the microbial ecology of this infection is complex since it contains members of the endogenous bacterial flora. Candida forms mixed biofilms with streptococci in the oral cavities of humans but the role of these biofilms during the course of mucosa-associated or invasive infections is unknown. In the proposed work C. albicans and S. oralis will be used as model organisms to study the mucosal disease-promoting interactions of the non-pathogenic oral bacterial flora with opportunistic fungi that have established virulence, such as C. albicans. Our studies rely upon a systematic and comprehensive approach to characterize both host- and pathogen-mediated aspects of the pathogenesis of mixed opportunistic mucosal infections. We hypothesize that enhanced proinflammatory host responses and increased Candida virulence mediate a pathogenic synergy of these microorganisms in the oral and esophageal mucosa. In aim I we will characterize the influence of streptococci on the TLR2/4-mediated host response to mixed infection and test the hypothesis that the exaggerated proinflammatory and neutrophilic response to mixed Candida-streptococcal biofilms leads to increased pathology. In aim II we will examine mechanisms of mixed microbial community assembly and virulence. The role of streptococcal glucosyltransferases, major enzymes contributing to the extracellular slime that promotes biofilms, will be examined. In C. albicans we will identify the transcription factor(s) tht control dual community assembly. We will also test the hypothesis that S. oralis increases the virulence of C. albicans by modulating several Rim101-dependent virulence genes, important in oral mucosal invasion, using a mouse oral co-infection model. An understanding of the effects of oral streptococci on Candida virulence and the mucosal inflammatory response will provide important novel insights into the pathogenesis of fungal infection within the oral and esophageal host niche. More importantly, the discovery of a synergistic role for oral bacteria in pathogenesis may have important clinical implications in the treatment of oral thrush which is currently based solely on antifungals.